It is often said that bilinguals are not two monolmguals in one person. But what does this really mean, especially in the context of bilingual acquisition? Despite the upsurge of case studies of bilingual children since the 1990's, the main central issue within the literature has largely remained focused on the one-vs. -two-system debate. Earlier studies focused on the question of whether bilingual children had a single/fused system or two separate/differentiated ones. There are a growing number of more recent studies focusing, instead, on the relationship between the two languages in the developing language system of the child. The study on which this thesis is based is the first longitudinal group study of lexical development of French-English bilingual children. The study aims to investigate the nature of the developing bilingual lexicon and its impact on the development of syntax. The key questions addressed in this new body of research include: are bilingual children developing in the same way and at the same rate as their monolingual peers; are there cross-linguistic influences on bilingual acquisition; are there features, patterns or processes specific to bilingual acquisition? We report findings from a longitudinal group study of 13 children between 1;4 and 3;0 who are acquiring French and English simultaneously within the one person - one language framework. The originality of this study lies in several main points. First of all, a larger number of children have been studied systematically than in traditional longitudinal studies, which are usually based on either cross-sectional sample or on single cases. Secondly, the children in this study have been systematically selected according to a set of sociolinguistic variables. This allows meaningful comparisons of the results as well as possible future replications of the study with even larger samples or with other language pairs. Furthermore, the methods used in the study are innovative in that both quantitative and qualitative methods have been used longitudinally as opposed to onLy longitudinal qualitative data or only quantitative cross-sectional data. The profiling of the bilingual lexicon reports that bilingual children's lexical categories in each language develop in a parallel manner whether or not the children are dominant in a language. The results also show that their development is very similar to previously reported data for monolingual children. Despite current theories,the evidence suggests that bilingual children produce translation equivalents before the 50-word stage. However, I attempt to bring forward the idea that cross-linguistic equivalents are different from synonyms within a language and so bilinguals cannot be compared to monolinguals in that respect. This thesis also sets the age of first word combinations for bilingual children to around 1 ;8 while claiming that this is only achieved after each language has reached the 50-word milestone. Finally, great variability is noted throughout the thesis in terms of lexical development amongst the children. Some of the differences are explained by socio-linguistic factors such as parental strategies and language exposure. Therefore, the importance of accounting for such factors when studying bilingual language development is underlined. Our understanding of bilingual acquisition centrally contributes to our understanding of language acquisition in general. Similar features of bilingual and monolingual acquisition have been highlighted throughout this thesis. Thus, the bilingual lexicon has shown to develop at a similar rate and in a similar manner as the monolingual one despite being strongly influenced by individual socio-linguistic factors.